Direct epoxidation of higher olefins (containing 3 or more carbon atoms) such as propylene with oxygen and hydrogen has been the focus of recent efforts. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing support (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium and a titanium zeolite (see, e.g., JP 4-352771).
Mixed catalyst systems for olefin epoxidation with hydrogen and oxygen have also been disclosed. For example, Example 13 of JP 4-352771 describes the use of a mixture of titanosilicate and Pd-on-carbon for propylene epoxidation. U.S. Pat. No. 6,008,388 describes a catalyst comprising a noble metal and a titanium or vanadium zeolite, but additionally teaches that the Pd can be incorporated into a support before mixing with the zeolite. The catalyst supports disclosed include silica, alumina, and activated carbon. U.S. Pat. No. 6,498,259 discloses the epoxidation of an olefin with hydrogen and oxygen in a solvent containing a buffer in the presence of a catalyst mixture containing a titanium zeolite and a noble metal catalyst. It is postulated that hydrogen peroxide, which is formed by reacting hydrogen and oxygen over the noble metal catalyst, reacts with the olefin in the presence of the titanium zeolite to form epoxide.
The direct epoxidation may be performed in a slurry or a fixed bed (see, e.g., U.S. Pat. No. 6,498,259). In a slurry process, catalyst particles of <100 μm are typically used; the separation of liquid product stream from catalyst particles by, e.g., filtration, is often troublesome. It is desirable, particularly in a fixed-bed process, to use catalysts of larger particle size, e.g., >0.5 mm, for the ease of separation and acceptable pressure drop across the bed.
Mixed catalyst systems have been used for other reactions. EP 0 211 985 discloses a process using a mixture of an olefin disproportionation catalyst and a double bond isomerization catalyst. Examples II and III of EP 0 211 985 illustrate that mixtures of the two catalysts with particles of 0.42 mm to 0.84 mm in diameter (20 to 40 mesh) give good activities.